The component I will analyze is the tower. The tower is the main structural component of the robot. It is directly attached to the skateboard and is completely immobile. It extends vertically with two posts and ends with a plate at the top to bind both of the posts. The tower acts as a guide rail and a stop for the accelerating masses.
The functional requirements of the tower are:
1. Strong and doesn’t deform under pressure
2. Lightweight
3. Rigid
4. Height has to be adjustable
The tower is mounted directly to the skateboard and needs to adequately support the mass at the top as well as the mass that is accelerating upwards. Originally the design had only one post, but in order to add more support we changed it to three poles in a triangular shape. This would create more resistance from bending and it would also create more surface area to mount the top of the tower to. The one downside to this design is that it requires extra weight to have three poles. As I thought more about this more I realized that the board will be constrained from rolling sideways and we only need two poles: one in front and one in back. These two poles will support each other if the nose of the board lands before the tail and vice versa. To further reduce the mass of the poles, I decided that we could achieve just has much strength with a hollow, wider rod than a solid thinner rod. With a hollow post, it will add a lot less weight to the board and therefore will require less energy to lift the board up.
To build this tower the proper material has to be selected. I have narrowed down the materials to aluminum and steel. Originally copper was also considered because it is commercially available as gas and water piping in houses. Although it is used a lot it is still more expensive than aluminum and weighs more. It is a stronger metal, but the weight difference and extra cost does not make it the most feasible material. In order to conserve mass I thought it was more practical to use an aluminum rod. Since aluminum is more expensive and not as strong it actually turns out that steel is a better choice. This is also especially true because hollow steel pipes are very available and inexpensive. Also because the wall of the steel is so thin, the mass added to the skateboard does not make a noticeable difference. Another advantage to using steel is that it is magnetic. This is important because part of our robot design requires magnets on the tower to catch the accelerating mass. With a magnetic material, these magnets can be placed on the very top of the structure and keep their magnetic field going through the steel without taking any impact from the mass that is accelerating towards it.
There are multiple ways to make the height of the tower adjustable. The ones under consideration are using a setscrew, a screw through the entire pipe, or a clamp. The setscrew idea is really easy to adjust and could make the tower immobile, but it might be at the expense of the cylindrical shape of the pipe. If the screw is tightened too much it will dent the pipe since it is hollow. Using a screw that goes through the pipe will not dent it and still allow for the mass to slide smoothly along it. With multiple holes drilled along the pipe the tower will be adjustable to certain heights. The downside to this is that it can compromise the integrity of the steel pipe if the hole is too big. The last idea is using a clamp that can be tightened around the pipe which is good because it can be moved anywhere along the pipe without prior holes drilled. One of the problems with this is that it has to be able to provide enough of a frictional force to prevent the tower from moving. Since metal does not have a big coefficient of friction the clamp would have to be made out of a material that does like plastic. In order to apply enough of a normal force there is a good chance that the plastic will fracture. Therefore I reason that it will work better using a screw that goes completely through the pipe.
After analyzing the tower, it is evident that it should be made out of two hollow steel pipes with a steel top that has a through screw to adjust the height. This design will provide the most cost effective part without taking away from the stability of it or adding too much more weight the robot will have to lift off the ground.
References:
1. www.matweb.com (for material properties)
2. www.mcmaster.com (for material prices)